In the course of various operations conducted in connection with exploration and recovery of hydrocarbons offshore and onshore, tools and different underwater equipment are employed which by means of an activation mechanism, such as electrical signals, hydraulics, pneumatics, explosive charges or the like, are controlled and operated from an inactive or closed position to an active or open position. Examples of such underwater equipment may be different types of valves, well plugs etc.
Since serious consequences may result with regard to both cost and the environment if a valve or a plug, for example, opens inadvertently, or fails to open when it should, it is vital for the activation mechanism to be reliable and to function properly.
As an example we refer to the well-known situation within the oil industry where for a variety of reasons a well or a formation in the well has to be shut down or leakage/pressure-tested during the start-up and/or service life of the well. This may occur, for example, when isolation has to be carried out between different zones in the well, when fluid has to be injected into the well, during perforation of pipes in the well, during cementation of the well and in a number of other operations. As a rule use is then made of one or more plugs (so-called well plugs) to perform this shutdown, where the plug(s) have to be able to withstand high pressure, high temperature in addition to a corrosive environment which may exist in such a well.
These plugs may either be retrievable or permanent, with the well conditions, the type of operation(s) to be conducted etc. determining which of these types of plug should be employed.
The retrievable plugs are recovered from the well after use by means of mechanical devices, which may, for example, be wirelines, “slick lines” or “coiled tubing”. These plugs, however, particularly if they are left for too long in the well, have a tendency to jam, or they may also become deformed on account of the great pressures to which they are exposed, with the result that they cannot be retrieved from the well without considerable effort.
When using permanent plugs, these may be completely or partially destroyed by means of various mechanisms. Plugs of this kind may be made of a soft or reactive material, such as rubber, composite materials etc., where the material may either be decomposed or perforated by suitable means, thereby admitting a through-flow through the pipe or the well. For example, after a pressure-testing of a well is completed, a chemical can be added to the well to destroy the rubber plug when the plug requires to be removed. There will, however, be a high degree of uncertainty associated with ascertaining when the plug has been “removed”, and whether it has been completely or only partially removed.
Permanent plugs may also be made of a brittle material, where the plug is smashed by means of suitable methods and mechanisms after the desired operation has been completed.
The use of plugs of this kind, which may be made of ceramic material, glass etc., is well known, and glass in particular is considered to be highly suitable within the oil industry. Glass is almost inert with respect to all types of chemicals and is safe for personnel handling the plug. The glass's properties also enable it to retain its strength at high temperatures and it can remain in an oil well for a very long time without being damaged or broken down.
In the known solutions a plug as mentioned above is removed by means of an explosive charge, with the result that the glass is smashed into small particles which are easily washed out of the well without leaving any remnants which may be harmful. These explosive charges can be incorporated in the actual plug, or mounted above the actual plug. The actual detonation is remotely controlled, and may be triggered from the surface of the well.
An example of a glass test plug, where the plug is arranged to be able to be removed by means of an explosive charge is known from NO B1 321.976. The plug comprises a number of layered or stratified ring discs of a given thickness, which are placed in abutment on top of one another. Between the different layers of the plug an intermediate film of plastic, felt or paper is inserted; the various glass layers may also be joined by means of lamination by an adhesive such as a glue. During use the plug will be mounted in a plug-receiving chamber in a tubing, where the underside of the plug rests in a seat at the bottom of the chamber. An explosive charge is furthermore incorporated in the top of the plug by one or more recesses being drilled out from the top of the plug, in which recesses the explosive charge(s) are placed.
The use of explosive charges for shattering test plugs can provide a safe and calculable removal of the plug. However, in many countries very stringent requirements are placed on the use and import of explosives, and it is therefore desirable to provide a solution where the test plug can be controllably removed without the use of such means.